IPv4 addressing & subnet masks
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Hi Don & team,
Mostly just wanted to say thanks for episode #00010010 of 70-410. Over the years I've thought "I should try to understand what this subnet thing really means" and have looked into it briefly at various times, but I think I finally 'get it' after your excellent explanation.
Also, just wanted to confirm something to make sure I'm clear about it...
In the show you say "Windows will support all those subnet masks in-between the default classful subnets." If I'm understanding it correctly though, there are only 9 possible valid values for a subnet mask octet:
11111111 - 255
11111110 - 254
11111100 - 252
11111000 - 248
11110000 - 240
11100000 - 224
11000000 - 192
10000000 - 128
00000000 - 0as something like:
236 - 11101100
for example, wouldn't make any sense as a subnet mask. Is this correct, or am I missing something?
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That is exactly right. There are a limited number of subnet masks since there are only 32 bits available to work with and the ones and the zeros cannot be mixed. A full list would look something like this:
0.0.0.0 - 00000000.00000000.00000000.00000000 - /0 128.0.0.0 - 10000000.00000000.00000000.00000000 - /1 192.0.0.0 - 11000000.00000000.00000000.00000000 - /2 224.0.0.0 - 11100000.00000000.00000000.00000000 - /3 240.0.0.0 - 11110000.00000000.00000000.00000000 - /4 248.0.0.0 - 11111000.00000000.00000000.00000000 - /5 252.0.0.0 - 11111100.00000000.00000000.00000000 - /6 254.0.0.0 - 11111110.00000000.00000000.00000000 - /7 255.0.0.0 - 11111111.00000000.00000000.00000000 - /8 255.128.0.0 - 11111111.10000000.00000000.00000000 - /9 255.192.0.0 - 11111111.11000000.00000000.00000000 - /10 255.224.0.0 - 11111111.11100000.00000000.00000000 - /11 255.240.0.0 - 11111111.11110000.00000000.00000000 - /12 255.248.0.0 - 11111111.11111000.00000000.00000000 - /13 255.252.0.0 - 11111111.11111100.00000000.00000000 - /14 255.254.0.0 - 11111111.11111110.00000000.00000000 - /15 255.255.0.0 - 11111111.11111111.00000000.00000000 - /16 255.255.128.0 - 11111111.11111111.10000000.00000000 - /17 255.255.192.0 - 11111111.11111111.11000000.00000000 - /18 255.255.224.0 - 11111111.11111111.11100000.00000000 - /19 255.255.240.0 - 11111111.11111111.11110000.00000000 - /20 255.255.248.0 - 11111111.11111111.11111000.00000000 - /21 255.255.252.0 - 11111111.11111111.11111100.00000000 - /22 255.255.254.0 - 11111111.11111111.11111110.00000000 - /23 255.255.255.0 - 11111111.11111111.11111111.00000000 - /24 255.255.255.128 - 11111111.11111111.11111111.10000000 - /25 255.255.255.192 - 11111111.11111111.11111111.11000000 - /26 255.255.255.224 - 11111111.11111111.11111111.11100000 - /27 255.255.255.240 - 11111111.11111111.11111111.11110000 - /28 255.255.255.248 - 11111111.11111111.11111111.11111000 - /29 255.255.255.252 - 11111111.11111111.11111111.11111100 - /30 255.255.255.254 - 11111111.11111111.11111111.11111110 - /31 255.255.255.255 - 11111111.11111111.11111111.11111111 - /32
/0 and /32 are only for special use cases so that really just leaves 31 different subnet masks you should encounter out and about.
Thanks for writing in,
Don Pezet
Host, ITProTV -
Great, thanks for the info!